Materials of electrically conductive polymer can be based on the mixture of polymer matrix and conductive particles embedded into the matrix or into an inherently conductive polymer.
In the former case the polymer matrix can be an adhesive and the electrically conductive particles metal or metal oxide or carbon particles such as carbon nanotubes (CNTs). The materials can also be directionally conductive. An electrically conductive material will usually also be thermally conductive. Wiedemann-Franz law states that the ratio of the electronic contribution to the thermal conductivity and the electrical conductivity of a metal is proportional to the temperature. For other materials the relationship is more complex.
The electrically conductive polymer films are usually produced by mixing the filler material with a polymer resin and in order to have a conductive mixture the amount of filler material shall exceed the percolation threshold. Mixed systems have limited lifetime and must be remixed prior to use.
In order to increase signal transmission capability without having to increase the amount of conductive filler material conductive films are made anisotropic. Anisotropic films can also be designed so that they have insulating properties in certain directions.
In EP 1809716 is described a method for making a directionally conductive adhesive based on CNTs. A tape having an insulation base and a parallel arrangement of CNTs acting as electrical contact points is made by growing carbon CNTs on a material used in the tape or arranging CNTs on the tape before adding the adhesive part to the tape.
In U.S. Pat. No. 5,429,701 is described how electric interconnection between discreet individual conductors of soft magnetic metal in two layers is achieved by adjoining the conductors by a conductive adhesive. The adhesive have particles of soft magnetic metal and by applying a magnetic field the particles can be gathered in an area between the conductors.
It is known that dipolar rigid asymmetric particles or molecules can be aligned by an electric field; this is especially used for small molecular weight liquid crystals.
In these cases material having permanent dipole moments is fluid in normal conditions, which makes electric field alignment possible.
Aligned structures of infusible conductive carbon particles, like CNTs, are known to be formed by chemical vapour deposition or spinning.
A method for the directional growth of CNTs is shown in U.S. Pat. No. 6,837,928. CNTs are grown in an electric field that directs their growth and thus leads to aligned CNTs when the growing procedure is completed.
Electric field alignment of carbon nanocone (CNC) material has been demonstrated in Svåsand et al. Colloids & Surf. A Physicochem. Eng. Aspects 2007 308, 67 and 2009 339 211. In these articles it is shown that nanocone material dispersed in silicon oil can form micron size nanocone “fibres” when a field of minimum 50 V/mm is applied. In order to form fibres within a reasonable time fields of 400V/mm is used.
In Schwarz et al. Polymer 2002, 43, 3079 “Alternating electric field induced agglomeration of carbon black filled resins” is reported how carbon black filled resins below zero-field percolation threshold can form electrically conductive networks when a field of 400 V/cm is applied between copper electrodes dipped into the resin. This result has been reproduced by Prasse et al. Compos. Sci. Tech. 2003, 63, 1835.
US 20090038832 describes a method for forming an electrical path having a desired resistance from a mixture of carbon and metallic nanotubes dispersed in a curable polymer matrix. Electrodes are placed in contact with the dispersion and electrical energy is applied until the desired electrical resistance is reached. A pure semi-conducting connection can be achieved by burning away metallic nanotubes that may be part of the carbon nanotube mixture, by applying a current after the deposition. The polymer matrix is cured in order to fix the device. Essentially same result has been achieved using copper particles in US20030102154A1.
A disadvantage with the method is that carbon nanotubes are very expensive and difficult to produce on an industrial scale. A dispersion of nanotubes is difficult to store and require specific manufacturing steps like homogination or sonication prior to application of the dispersion to the substrate and electrode.
These methods are dedicated to the use of microelectronics and circuit boards. Moreover, they aim at connecting the alignment electrodes so that the alignment is a step-wise procedure where the alignment electrodes are connected to the material and remain in the end product.